1. Field of the Invention
This invention relates in general to a method of processing motion picture film or video tape to reduce strobing effect and to increase resolution and density.
2. Description of the Prior Art
Prior to the introduction of sound to motion pictures, when it became necessary to adopt a standard frame rate to synchronize the moving image with the accompanying sound, motion pictures were hand cranked at varying frame rates, from as slow as ten frames-per-second (fps) to as high as 20 fps. At the slower frame rates almost any motion, such as walking, appeared jerky and unnatural because to wide a range of motion occurred in too few frames. The fewer frames-per-second allowed for a proportionate increase in exposure time, thus producing a higher resolution image.
One solution to this problem was offered in U.S. Pat. No. 1,815,455, issued Jul. 21, 1931, F. Waller. Waller superimposed multiple preceding and succeeding images, from as few as three to as many as five, on one frame. The preceding images and succeeding images were exposed in differing amounts less than the normal exposure time. The total of the partial exposures equalled one normal exposure. This was particularly applicable for creating motion pictures from a series of still pictures or drawings showing successive phases of action. In the time period following the filing date of the Waller patent, sound films began to appear and the problem Waller described for motion pictures was solved by the higher films speed (i.e., 24 fps) adopted as a standard for sound-synchronized film.
One problem which Waller did not address and the higher film speed did not cure was the strobing effect. The strobing effect is an undesirable optical illusion resulting from the relationship between the speed of the motion of the photographed subject and the time interval between camera exposures. The most common example is the illusion of a spoked-wheel appearing to revolve backwards when in reality it is moving forward.
A spoked-wheel rotating in a series of motion picture frames may appear in three phases: moving forward (in the direction of rotation); moving backward (opposite the direction of rotation); and standing still (no apparent rotation).
When the wheel appears to be moving forward, the position of the spokes, when photographed, has rotated more than a number of complete revolutions (i.e., 360 degrees) divided by the number of the spokes, so that the image in each succeeding frame has advanced from its position in the preceding frame.
To create the optical illusion of moving backward, the position of the spokes, as photographed, has rotated less than a number of complete revolutions divided by the number of spokes, thus the image in each succeeding frame has retrograded from its position in the preceding frame. A wheel that appears to be standing still is rotating a number of complete revolutions divided by the number of spokes, thus appearing to be in the same position in each succeeding frame. If a light-colored wheel is rotating against a dark background, each image of the wheel produces a contrasting area of brightness in the position of the image and no light (darkness) where the image is absent. The opposite would be true of a dark wheel rotating against a light background.
The higher the resolution of the spoke, the more pronounced is the strobing effect. Thus the strobing effect is more pronounced if the images overlap one another, intensifying the portion of the overlapped images, where the brightness contrast (in the case of a light wheel before a dark background) is greater.
In the early days of black and white films, fine-grained film emulsions, as slow as 5 ASA to 15 ASA, combined with slower film speeds and longer exposures, produced high-resolution images. Today, largely because of improved optics, particularly the zoom-lens, and the desire to remove film-making from the studio environment, the use of high-speed films, to 400 ASA and higher, has reduced the resolution of the image in motion pictures.
Density refers to the amount of information contained in a particular image measured by the number of individual elements comprising the total picture. The greater number of individual elements, the greater the density. Because grain structure of an emulsion is randomly distributed, the grain structure varies between individual frames. Since the amount of grain structure determines the density of the emulsion, and the thickness of the emulsion determines the film speed, the film maker must choose between high density and low speeds or low density and high speeds.
The problem of strobing and reduced resolution and intensity also occurs in video taping. In video taping, the image is electronically scanned and recorded on a medium, usually video tape. The word "film" will be used herein to refer to both photographic emulsion film and video tape.